Cross-Linked Polyphosphazene Blends as Robust CO₂ Separation Membranes

An effective cross-linking technique allows a viscous and highly gas-permeable hydrophilic polyphosphazene to be cast as solid membrane films. By judicious blending with other polyphosphazenes to improve the mechanical properties, a membrane exhibiting the highest CO₂ permeability (610 barrer) among polyphosphazenes combined with a good CO₂/N₂ selectivity (35) was synthesized and described here. The material demonstrates performance stability after 500 h of exposure to a coal-fired power plant flue gas, making it attractive for use in carbon capture applications. Its CO₂/N₂ selectivity under conditions up to full humidity is also stable, and although the gas permeability does decline, the performance is fully recovered upon drying. The high molecular weight of these heteropolymers also allows them to be cast as a thin selective layer on an asymmetric porous membrane, yielding a CO₂ permeance of 1200 GPU and a CO₂/N₂ pure gas selectivity of 31, which does not decline over 2000 h. In addition to gas separation membranes, this cross-linked polyphosphazene can potentially be extended to other applications, such as drug delivery or proton exchange membranes, which take advantage of the polyphosphazene’s versatile chemistry.